1. Field of the Invention
The present invention relates to a fail detecting apparatus for a vehicular automatic transmission, and more particularly to a fail detecting apparatus which detects failure of oil pressure switches which detect operating oil pressure of friction elements.
2. Description of the Prior Art
Vehicular automatic transmissions are equipped with a plurality of friction elements to realize a plurality of gear positions by engagement and disengagement of the friction elements in combination. Oil pressure switches which detect operating pressure of the respective friction elements are provided in a hydraulic circuit and used for feedback-control of operating pressure.
On the other hand, when a predetermined friction element set in advance is engaged and another friction element is additionally engaged, their simultaneous engagement causes interlock. Thus, in the case where a combination of friction elements to which supply of oil pressure has been detected by oil pressure switches causes interlock, the shift stage is fixed at a gear position designated in advance so as to avoid interlock.
However, if an electric system such as oil pressure switches has failed, it is difficult to properly control operating pressure, and in particular, interlock is erroneously detected, making it necessary to carry out control which is unnecessary under normal conditions. To avoid this, it is necessary to check whether or not oil pressure switches are normal.
To this end, the applicant of the present application has proposed in Kohyo No. WO2003/029700 an automatic transmission control apparatus which determines that an electric failure has occurred if, in a state in which the gear position is being kept at any gear position, a friction element engaged at this gear position and a friction element to which supply of oil pressure has been detected by an oil pressure switch do not coincide.
With this arrangement, even when an electric failure occurs, no measures to avoid interlock are taken, so that normal control can be continued.
However, the above conventional automatic transmission control apparatus has the problem that, although it is possible to detect the occurrence of an electric failure, it is difficult to determine whether an oil pressure switch has failed or a hydraulic control valve using a solenoid or the like in a hydraulic circuit has malfunctioned, because there are various conditions on which a friction element engaged at the present gear position and a friction elements of which engagement has been detected by an oil pressure switch do not coincide.